ABSTRACT

Middle Triassic to Lower Jurassic continental conglomerate, sandstone, and mudrock are widely exposed in the highest tectonostratigraphic terranes (internal domains) of western and central circum-Mediterranean orogenic belts. These red beds are a key tectonic facies assemblage representing onset of the Tethyan rift-valley stage in the western Mediterranean, during which plate and microplate boundaries were localized during breakup of Pangea. These red beds likely define the boundary of the Mesomediterranean microplate, which played a key role in the Cenozoic evolution of the western Mediterranean. Red beds unconformably overlie Paleozoic metasedimentary and locally plutonic rocks, and they are covered by Early Jurassic and younger sedimentary strata; they are generally mildly deformed and only locally metamorphosed.

Sandstone detrital modes vary from quartzarenite to quartzolithic, reflecting a provenance from Cambrian–Carboniferous metasedimentary rocks similar to those underlying the red beds. Reevaluation of previously published petrographic databases and detailed hot-cathodoluminescence (H-CL) analysis of quartz grains indicate that most quartz grains were derived from heterogeneous metamorphic, plutonic, and volcanic rocks. Evaluation of the diagenetic evolution of red beds using chemical-mineralogical analyses and H-CL analyses indicates that compaction and cementation played key roles. Compaction consists of brittle deformation, with breakage of quartz grains and production of early quartz cement, which closed the fractures, followed by a second stage of quartz cementation. Carbonate cements consist of dolomite, ankerite, and calcite. This last cement, related to dedolomitization/calcitization processes, produced carbonate crystals with iron oxides. The sandstone experienced intense reduction of intergranular volume during early stages of burial, as indicated by contrasting compactional porosity loss (COPL; mean of 31.1%) versus cementational porosity loss (CEPL; mean of 8.5%). These data demonstrate the minor role of cementation in reducing porosity and the prevalence of compaction as the main process destroying primary pores. The diagenesis of the analyzed red beds is variable within several internal domains of the orogenic belts, suggesting local influences related to the provenance and geotectonic evolution of each basin.

INTRODUCTION

The Anisian–Carnian Verrucano Group of the Tuscan metamorphic units and the Triassic–Lower Jurassic Pseudoverrucano Formation of the homonymous unit are mainly continental red beds at the base of the Alpine orogenic cycle in Tuscany (e.g., Perrone et al., 2006, and references therein). Middle Triassic–lowest Jurassic continental red beds occur in the internal units of western-central Mediterranean thrust belts (from the Apennine to the northern Africa and Betic chains); they are similar to the metamorphic Verrucano and to unmetamorphosed Pseudoverrucano units, representing key stratigraphic markers characterizing the sedimentary evolution of continental basins that developed in early stages of western Tethyan rifting due to breakup of Pangea (e.g., Perrone et al., 2006; Critelli et al., 2008; Zaghloul et al., 2010; Perri et al., 2013). The Verrucano and Pseudoverrucano successions developed during continental rifting of Pangea, which led to later breakup at the edges of a future microplate surrounded by the Europe, Africa, and Adria-Apulia plates. These red beds contrast with Triassic continental red beds (the Germanic-Andaluse red beds) from the European, Iberian, and African landmasses.

It is well known that diagenetic processes are controlled by several parameters intimately related to: (1) the depositional realm (eodiagenesis) and (2) the evolution of the sedimentary basin during burial (mesodiagenesis). In addition, during basin uplift, sediment is prone to further alterations (telodiagenesis; i.e., Worden and Burley, 2003). Therefore, diagenetic processes reflect postdepositional evolution of sediments, closely connected to geotectonic scenarios (i.e., Dickinson, 1985). The composition of sediments influences processes during early stages of diagenesis (Wilson, 1994); subsequent burial processes are influenced by subsidence history, pressure-temperature (P-T) conditions, and interstitial water composition (i.e., Wilson, 1994; Worden and Burley, 2003; Allen and Allen, 2013; Perri et al., 2016). Therefore, diagenetic products represent important features that can be used to unravel the evolution of geotectonic histories.

Starting in the Middle Jurassic, this microplate was independent of the main plates in the Mediterranean area (i.e., Europa-Iberia and Africa-Adria plates). Thus, the Pseudoverrucano and Verrucano red beds were the Triassic–earliest Jurassic precursors of a mid-Jurassic–early Miocene continental block (Mesomediterranean microplate), from which the internal domains of the western Mediterranean Alpine chains originated (Fig. 1; Critelli et al., 2008, 2017; Perri et al., 2017, Critelli, 2018). The key outcrops of the Pseudoverrucano red beds, discussed in this paper, are mainly located in the Betic Cordillera of southern Spain (Malaguide Complex), the Rif Chain in Morocco (Ghomaride Complex), and the southern part of the Calabrian terranes in southern Italy (Longi-Taormina Unit).

The borderland of the Alboran Sea has two main thrust belts, represented by the Betic Cordillera of southern Spain and the Rif Chain of northern Morocco (Fig. 1). The internal domains of the Betic-Rif belts consist of the Nevado-Filabride, Alpujarride, Malaguide, “Dorsale Calcaire,” Sebtide, and Ghomaride Complexes (Durand Delga and Fontboté, 1980). The Malaguide (southern Spain) and Ghomaride (northern Morocco) Complexes include Paleozoic metasedimentary basement, mainly composed of slate, phyllite, meta-arenite, and marble, unconformably overlain by Mesozoic through Cenozoic strata with Triassic continental red beds at the base (Pseudoverrucano-type; Perrone et al., 2006). The Malaguide and Ghomaride Triassic red beds have been formally grouped into the Saladilla Formation, as defined by Roep (1972) in the eastern Betic Cordillera and by Martín-Algarra (1987), Martín-Algarra et al. (1995), and Maate (1996) in the western Betic Cordillera, and the Ghomaride Complex of the Rif chain in Morocco. Small outcrops of Pseudoverrucano red beds also occur in the Tell chain in Algeria. Toward the central Mediterranean, red beds occur in Calabrian terranes and in Tuscany in the Northern Apennines.

Pseudoverrucano sandstone ranges from quartzolithic to quartzarenite (Fig. 3). Monocrystalline quartz grains are the main type of framework grain; polycrystalline quartz with foliate fabric derived from metasedimentary source rocks and nonfoliated polycrystalline quartz are also common. The latter were mostly derived from siliceous sedimentary rocks and felsitic volcanic groundmass fragments. K-feldspar and plagioclase are minor or absent. Aphanitic lithic fragments include mainly low-grade metamorphic rocks (phyllite and slate); felsitic volcanic and radiolarian chert lithic fragments and a few extrabasinal carbonate grains are minor components (Fig. 3). Sandstone has been deeply modified by latediagenetic effects that produced intense dissolution of framework grains and pressure solution, with precipitation of authigenic minerals, and compaction causing severe reduction of porosity, (e.g., Critelli et al., 2008; Zaghloul et al., 2010; Perri et al., 2013). Quartzose to quartzolithic sandstone represents continental-block and recycled-orogenic provenance (Fig. 3), made up mainly of Paleozoic metasedimentary rocks similar to those underlying the red beds (e.g., Zuffa et al., 1980; Critelli et al., 2008). The similarity in composition, sedimentology, and diagenetic evolution of the red beds in different sectors of the western Mediterranean orogens suggests deposition in a distinctive Mesozoic belt (Perrone et al., 2006; Critelli et al., 2008). The red beds were deposited on a block of Variscan continental crust (Fig. 2), which had a central mountainous area that provided terrigenous sediments to surrounding intracontinental rift basins. Following deep erosion during the Triassic, the mountainous areas were transformed to a peneplaned area of low relief, whereas the former continental basins evolved to neritic carbonate basins during the Early Jurassic. The Triassic parts of the Pseudoverrucano-Verrucano successions were surrounded by the Germanic-Andaluse facies, now cropping out on the Iberian and African plates, and eastward, the Tethyan marine domains (Fig. 2).

DIAGENESIS

Sandstone Characteristics

Several diagenetic processes can be distinguished and grouped into compaction, cementation, and mineral transformation. The studied sandstone shows abundant evidence of intense compaction, dramatically reducing the IGV to values lower than 20%. Mechanical compaction is manifested by deformation of labile grains, mainly phyllite, silty clay intraclasts (rip-up clasts), and micas. Deformation and disaggregation of fine-grained polymineralic constituents produced pseudomatrix during early burial. The scarcity of these soft grains (mean of less than 8% of total framework components) resulted in low reduction of the original porosity by mechanical compaction. Following the model of Rittenhouse (1971), this reduction can be estimated as representing as much as 5% of the original porosity. In addition, intense fracturing of quartz grains produced dense packing by brittle deformation, as observed in H-CL images (Fig. 4). Chemical compaction is the most important primary porosity-reduction process through pressure solution between quartz grains, producing grain-to-grain concave-convex and long contacts. The ubiquitous presence of these contact types in all analyzed samples suggests a vertical reduction of framework by pressure solution. Based on criteria of Mitra and Beard (1980), an estimation of 15%–30% of vertical shortening of framework grains results in a porosity loss of 8%–14%. This process is favored by the mature composition of the framework; its effects are increased locally by the presence of thin clay films (illite or smectite) generated during early stages of diagenesis (Weyl, 1959; Wilson, 1994).

General view of quartzose framework of Pseudoverrucano sandstone in (A) polarized light, (B) cross-polarized light, and (C) hot luminescence. Intense packing of framework due to brittle deformation, and abundant pressure-solution contacts outlined by thin Fe-oxides and clay-mineral coatings (A and B) are evident. Colors in C represent quartz grains with different origins (red—volcanic; violet and dark blue—plutonic; dark brown—metamorphic). Note in C that thin bright blue corresponds with early syntaxial quartz cement coating grains and restored quartz grain fractures. A second stage of quartz cementation is indicated by dark-blue color.

Several mineral species have been identified as authigenic cement. Fe-oxides can be considered the earliest precipitation phase during burial. This cement appears as discontinuous opaque coats around grains (Fig. 5A), suggesting their precipitation was even earlier than final deposition of the sediment, acquired during transport or storage in intermediate subenvironments (Wilson, 1992). The presence of these coats did not prevent later syntaxial quartz overgrowth. In association with early stages of diagenesis, ferruginous ghosts of euhedral rhombic sections are common on grain surfaces (Fig. 5B). These features could be related to an early phase of siderite(?) mineralization (e.g., Morad, 1998). Locally (in the Gibraltar arc area), K-feldspar and albite overgrowths have been observed around scarce detrital feldspar grains (Zaghloul et al., 2010; Perri et al., 2013). Clay-mineral cementation mainly consists of kaolinite pore fillings (Fig. 5C), which can be considered an early process, postdating chemical compaction and quartz cementation. Kaolinite pore-filling aggregates are commonly deformed; platelets appear cemented by quartz (Fig. 5C). Syntaxial quartz overgrowth represents the more abundant cement in these samples (3%–8% of total rock volume; Fig. 5A). This cement was likely generated during chemical compaction as a consequence of pressure solution and restoration of fractured brittle quartz clastic components (Fig. 5E). Quartz cement fills pores, occluding the total effective porosity. As an initial phase of growth, a mosaic of microquartz was formed around quartz grains, as illustrated by SEM (Fig. 5D). Carbonate cements found locally, mainly in Spain and Morocco (Zaghloul et al., 2010; Perri et al., 2013), consist of single dolomite, calcite, or minor ankerite crystals (Fig. 6A), showing corrosion textures over quartz cement (Fig. 6B). Calcite mosaics consist of a few crystals with abundant Fe-oxide impurities (Fig. 6B). This cement likely formed as the product of calcitization of Fe-carbonate (Fe-dolomite, ankerite?), which had previously formed during deep burial (Morad et al., 1994). Latedigenetic Fe-oxides appear occluding inner pore areas, secondary pores, and microporosity in kaolinite aggregates, representing the last phase of cementation.

The studied sandstone shows several mineral transformations, including kaolinitization of feldspars and lithics during very early stages of diagenesis, as indicated by the presence of kaolinitized deformed grains (Fig. 7A). Kaolinite platelets have been transformed into dickite, acquiring block shapes (Figs. 7B and 7C). Illite coatings (Fig. 7D) can be related to transformation of a smectitic precursor (e.g., Worden and Morad, 2003) generated during early diagenesis. Late illitization, as observed mainly in Spain and Morocco, consists of whisker and ribbonlike aggregates filling residual pores (Fig. 7E). Calcitization of Fe-carbonates (early siderites and late Fe-dolomite and ankerite) can be deduced by the presence of euhedral ghost textures and impurities of Fe-oxides, respectively (Figs. 5B and 6B).

Evaluation of Silica Production: Export versus Import

The studied sandstone shows evidence of significant silica activity. Pressure solution produced abundant silica that was incorporated into diagenetic waters and was then precipitated as quartz overgrowth cement. Mitra and Beard’s (1980) models permit an evaluation of the silica mass balance. The balance between silica production by pressure solution and quartz cementation suggests an equilibrium without significant silica import or export. However, some samples display high IGV values that may suggest local additional import of silica from external sources during early stages of mesodiagenesis. The fact that these samples appear ubiquitously without clear spatial control suggests that additional silica could be related to clay-mineral transformations in interbedded silty-clay deposits (e.g., Boles and Franks, 1979; Sullivan and McBride, 1991; Morad et al., 2000; Worden and Morad, 2000). However, other external sources, such as volcanic activity, cannot be excluded. The presence of abundant volcanic quartz, as identified by H-CL, supports this possibility.

DISCUSSION

Diagenetic Model: Chronology of Processes

A temporal distribution of diagenetic processes can be established based on the textural relationships between detrital and intergranular minerals, being grouped into eo-, meso-, and telodiagenetic stages (Fig. 8B). During the eodiagenetic stage, Fe-oxides precipitated around framework grains under oxidizing conditions (Fig. 5A). Kaolinitization of scarce alumino-silicate framework components took place (Fig. 7A), suggesting phreatic fluxes of undersaturated waters. Locally, smectitic clay coats precipitated in association with illuviation processes within vadose/phreatic subenvironments from supersaturated water fluxes (e.g., Bjørlykke and Aagaard, 1992; Worden and Morad, 2003), constituting the precursor mineral for a later transformation to illite (Fig. 7D). Local alkaline waters favored growth of K-feldspar and albite as syntaxial rims, as described in Zaghloul et al. (2010). Finally, the presence of euhedral Fe-oxides around quartz grains suggests that Fe-bearing carbonate formed under reducing geochemical conditions (Fig. 5B; i.e., Morad, 1998; Henares et al., 2016). All these processes acted at low temperature and in an open chemical diagenetic system (Wilson, 1994), contemporaneously with mechanical compaction and development of pseudomatrix.

Mesodiagenetic processes are related to quartz cementation, mainly due to pressure solution of quartz grains. This cementation occurred after intense brittle deformation of the quartzose framework, which increased reactant quartz surfaces favoring cementation. The bright-blue and dark-blue colors of intergranular quartz in H-CL images suggest that two phases of quartz cementation took place (Fig. 5E). The blue color of quartz under H-CL may suggest a high temperature of crystallization (i.e., hydrothermal). The quartz cement volume in sandstone (2.65%–7.76% of total rock volume) is consistent with deduced vertical shortening (15%–30%) by pressure solution, based on theoretical models (see fig. 6inMitra and Beard, 1980). During mesodiagenesis, local precipitation of Fe-bearing carbonates represents the last cementation/replacement process in deepburial reducing environments. The mesodiagenetic evolution of clay minerals was manifested by transformation of smectites into illite by an increase in temperature (higher than 90 °C) during burial (e.g., Worden and Morad, 2003). In addition, platelets of kaolinite were transformed into blocky dickite polymorphs. The coexistence of both polymorphs suggests that temperature during diagenesis did not rise higher than 130 °C (Beaufort et al., 1998; Cassagnabère, 1998; Worden and Morad, 2003). In Sicily, transformation of kaolinite into illite has not been reported, suggesting low-K activity during burial.

Contrast with Interbedded Mudrock

The severe diagenetic effects on sandstones from continental red beds are confirmed by mineralogical analyses of the clay fraction in interbedded mudrock (see Perri, 2008; Perri et al., 2008a, 2011, 2013). This fraction is mainly characterized by a decrease of illite-smectite (I-S) mixed-layer percentages and an increased content of illitic layers in I-S at the base of the red bed sequence (e.g., Perri et al., 2008a, 2011, 2013). The percentage of illite (%I) of the I-S mixed layer was determined on the spectrum of the glycolated specimen according to Moore and Reynolds (1997), whereas the illite crystallinity (IC) value was measured on both air-dried and ethylene-glycol–solvated oriented mounts and calibrated using the standards of Warr and Rice (1994). IC values of 0.6°–0.7° Δ2θ are compatible with conditions at the upper limit of late diagenesis and suggest an estimated temperature of 100–160 °C. Thus, paleotemperature estimates indicate diagenetic/tectonic evolution corresponding to ~4–6 km of lithostatic/tectonic loading (Perri, 2008; Perri et al., 2008a, 2011, 2013).

Significance of Local Diagenetic Histories

Burial histories deduced from paragenetic sequences of diagenetic processes from red bed sandstone contrast slightly in the analyzed areas of the belt (Fig. 8A; Table 1). Early diagenetic (eodiagenetic) processes vary from the Betic and Rif belts to the Calabrian and Apennine belt. Sandstone of the Malaguide and Ghomaride red beds (Spain and Morocco, respectively; Fig. 8A; Table 1) is characterized by development of K-rich overgrowth cements on feldspars and I-S minerals coating framework grains. These cements are closely related to the compositions of source areas, which supplied enough K (i.e., K-feldspars) to increase its activity in diagenetic waters. In contrast, Calabrian Pseudoverrucano sandstone is K-feldspar free, and K diagenetic products are scarce (Table 1). Nevertheless, some eodiagenetic processes are common to the three studied areas, mainly related to intense oxidizing and undersaturated waters, as kaolinitization processes (transformation of alumino-silicates and pore-filling cements), and Fe-oxide coatings on framework grains, reflecting a common arid to semiarid climate during deposition. More intense mechanical compaction in Pseudoverrucano sandstone from the Peloritani Mountains is due to the greater abundance of fine-grained components (metamorphic and metasedimentary constituents, and rip-up clasts), which generated significant pseudomatrix. As a consequence, differences in diagenesis between these areas during the initial stages of burial were mainly governed by the original composition of the sediments, and thus by local provenance. Mesodiagenetic processes are similar in all studied areas, differing mainly by the grade of intensity at which they acted. Intensity of pressure solution was greater in some samples from Sicily and Spain, recording the lowest IGV values (Table 1). In contrast, greater IGV values (less-intense pressure solution) correspond with sandstone with more abundant quartz cement (i.e., Morocco). This fact suggests that additional and external silica sources could have produced quartz cement early in burial, thus preventing intense pressure solution. External silica sources could have been associated with volcanism and silica-rich hydrothermal solutions, and the interplay of these factors produced diverse IGV values. Other, deeper mesodiagenetic cement/replacement mineral phases, such as Fe-carbonates, developed mainly in western areas of the Betic and Rif chains.

CONCLUSIONS

Analysis of diagenetic processes within Middle Triassic–lowest Jurassic Pseudoverrucano sandstone along the western Mediterranean orogenic belts, including the Betic Cordillera, northern Africa Rif and Tell chains, and the Apennines, documents several distinct diagenetic characteristics. In spite of constituting a single depositional domain related to erosion of a continental Mesomediterranean landmass (Perrone et al., 2006; Critelli et al., 2008), the diagenetic history in each area differs slightly. Differences can be identified during the initial stage of diagenesis (eodiagenesis), intimately related to the composition of local sediment sources. Thus, western areas of the belt were influenced by K-bearing sources, producing K-bearing diagenetic minerals. Early diagenesis in the eastern areas of the belt was characterized by greater supplies of fine-grained lithic grains, favoring mechanical compaction. Mesodiagenetic (deep burial) processes are almost equivalent among areas; differences can be related to local supplies of silica-rich diagenetic waters in the central area of the belt, probably hydrothermal in origin. Other mesodiagenetic differences among areas suggest variations in geotectonic evolution, including onset of sediment deposition related to the Mesomediterranean microplate, its destruction during Alpine tectogenesis, and its incorporation into the internal units of western Mediterranean Alpine chains (Perrone et al., 2006). The results of this paper concur with those obtained in previous works on clay-mineral diagenesis in Pseudoverrucano interbedded mudrock (Perri, 2008; Perri et al., 2008a, 2011, 2013).

ACKNOWLEDGMENTS

This work was funded by the Ministero dell’Istruzione dell’Università e della Ricerca (MIUR) Progetti di Rilevante Interesse Nazionale (PRIN) 2001–2003 Project (Age and Sedimentary Characters of the Mesozoic Continental Red Beds [Verrucano] from Northern Apennines to the Betic Cordillera: Implications for Paleogeographic and Tectonic Evolution of the Central-Western Mediterranean Alpine Belts; Resp. S. Critelli, G. Mongelli, V. Perrone), MIUR-ex60% Projects (Paleogeographic and Paleotectonic Evolution of the Circum-Mediterranean Orogenic Belts, 2001–2005; and Relationships between Tectonic Accretion, Volcanism, and Clastic Sedimentation within the Circum-Mediterranean Orogenic Belts, 2006; Resp. S. Critelli), and the 2006–2008 MIUR-PRIN Project 2006.04.8397 (The Cenozoic Clastic Sedimentation within the Circum-Mediterranean Orogenic Belts: Implications for Paleogeographic and Paleotectonic Evolution; Resp. S. Critelli). Support for J. Herrero and J. Arribas was provided by the Spanish Dirección General de Investigación Científica y Técnica (DIGICYT) project CGL2014-52670-P and by the research group “Sedimentary Basin Analysis” UCM 910429. The authors appreciate the important support received by Jens Götze (Institut für Mineralogie, Freiberg, Germany) for treatment and interpretation of hot cathodoluminescence images. The authors are indebted to Ray Ingersoll, Tim Lawton, and William Cavazza for reviews and suggestions on an early version of the manuscript.

REFERENCES CITED

Allen

,

P.A.

, and

Allen

,

J.R.

,

2013

,

Basin Analysis: Principles and Application to Petroleum Play Assessment